Exhaust gas discharged in the process of burning fossil fuel contains NOx known to be a material causing acid rain and respiratory diseases. Therefore, in order to remove NOx from exhaust gas, selective catalytic reduction (SCR) using ammonia as a reductant is variously applied.
FIG. 1 shows a conventional system for denitrifying exhaust gas.
Referring to FIG. 1, the conventional system for denitrifying exhaust gas is configured such that urea water and air are injected into a reaction chamber 4 by an injector 34 to convert urea water into ammonia, NOx is removed using a catalyst charged in a reactor 5 to obtain an optimum denitrification efficiency, and environment pollution attributable to NOx or ammonia is effectively prevented. However, in the procedure of continuously discharging urea water into the reaction chamber 4 through an injection nozzle 341 of the injector 34, air introduced into the injector 34 by an air supply unit 31 is heated to high-temperature exhaust gas in the reaction chamber 4, so that the urea water introduced into the injector 34 is evaporated, with the result that the urea water is coagulated to clog the injection nozzle 341. Further, in the procedure of introducing urea water into the injector 34 by a urea water supply unit 32, the urea water has a predetermined viscosity and so the urea water does not pass through a urea water supply line 321 from the output end of a flow control pump 325 to the injector 34 and stays therein to form a dead zone. The urea water staying in the dead zone coagulates, and further coagulates during the procedure of continuously supplying the urea water into the injector 34, so that the urea water supply line 321 is clogged by the coagulated urea.